res_pjsip_sdp_rtp: implement hold state handling on moh_passthrough

When moh_passthrough is used, asterisk is only generating invites
of type sendonly and sendrecv instead of taking fully into account
the on hold state of the local and remote parties

ASTERISK-28738 #close

Change-Id: Iaaad9fbc033cb14803d433b8a4071bc337047761